title: Molecular basis of substrate-induced permeation by an amino acid antiporter.
abstract: Transporters of the amino acid, polyamine and organocation (APC) superfamily play essential roles in cell redox balance, cancer, and aminoacidurias. The bacterial L-arginine/agmatine antiporter, AdiC, is the main APC structural paradigm and shares the "5 + 5 inverted repeat" fold found in other families like the Na(+)-coupled neurotransmitter transporters. The available AdiC crystal structures capture two states of its transport cycle: the open-to-out apo and the outward-facing Arg(+)-bound occluded. However, the role of Arg(+) during the transition between these two states remains unknown. Here, we report the crystal structure at 3.0 Å resolution of an Arg(+)-bound AdiC mutant (N101A) in the open-to-out conformation, completing the picture of the major conformational states during the transport cycle of the 5 + 5 inverted repeat fold-transporters. The N101A structure is an intermediate state between the previous known AdiC conformations. The Arg(+)-guanidinium group in the current structure presents high mobility and delocalization, hampering substrate occlusion and resulting in a low translocation rate. Further analysis supports that proper coordination of this group with residues Asn101 and Trp293 is required to transit to the occluded state, providing the first clues on the molecular mechanism of substrate-induced fit in a 5 + 5 inverted repeat fold-transporter. The pseudosymmetry found between repeats in AdiC, and in all fold-related transporters, restraints the conformational changes, in particular the transmembrane helices rearrangements, which occur during the transport cycle. In AdiC these movements take place away from the dimer interface, explaining the independent functioning of each subunit.
authors.affiliation: Institute for Research in Biomedicine, 08028 Barcelona, Spain.
authors.forename: Lukasz
authors.initials: L
authors.lastname: Kowalczyk
authors.forename: Mercè
authors.initials: M
authors.lastname: Ratera
authors.forename: Antonella
authors.initials: A
authors.lastname: Paladino
authors.forename: Paola
authors.initials: P
authors.lastname: Bartoccioni
authors.forename: Ekaitz
authors.initials: E
authors.lastname: Errasti-Murugarren
authors.forename: Eva
authors.initials: E
authors.lastname: Valencia
authors.forename: Guillem
authors.initials: G
authors.lastname: Portella
authors.forename: Susanna
authors.initials: S
authors.lastname: Bial
authors.forename: Antonio
authors.initials: A
authors.lastname: Zorzano
authors.forename: Ignacio
authors.initials: I
authors.lastname: Fita
authors.forename: Modesto
authors.initials: M
authors.lastname: Orozco
authors.forename: Xavier
authors.initials: X
authors.lastname: Carpena
authors.forename: José Luis
authors.initials: JL
authors.lastname: Vázquez-Ibar
authors.forename: Manuel
authors.initials: M
authors.lastname: Palacín
mesh_terms: D000596:Amino Acids / Q000378:metabolism*; D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D011487:Protein Conformation
chemical_list: D000596:Amino Acids; D002352:Carrier Proteins